Without limiting the scope of the invention, its background is described in connection with electronic device packaging, as an example.
Modern electronic components utilize numerous integrated circuits. These integrated circuits are electrically connected to each other or to other electronic components. One method for connecting integrated circuits to electronic components utilizes an area array electronic package. The electronic connections between an integrated circuit packaged in an array package design and a printed circuit board (PCB) are typically composed of solder.
Integrated circuits may be connected to electronic components through a flip-chip electronic package design. The flip-chip electronic package is similar to the ball grid array electronic package in that solder balls are used to make a connection with other electronic components, such as a PCB. Solder balls are also used in a flip-chip design to attach the input and output ports of the substrate to the contact pads of the integrated circuit. As such, flip-chip packages do not require wire bonds. Solder balls or bumps may also be formed on the face of integrated circuits as they reside on semiconductor wafers before being sawed into individual dies.
In a typical Chip Scale Package (CSP) electronic packaging design, a previously bumped wafer (or die) is attached to contact pads on one face of an interposer or substrate. In a subsequent step, another set of solder spheres, typically larger in size, are attached to contact pads located on the opposite face of the interposer or substrate. These processes can be performed either in wafer form or on individual dies. The attachment of the larger spheres completes the package assembly and the wafer is ready for sawing into singular dies. The larger spheres of the finished package are commonly attached to another electronic component such as a printed circuit board (PCB). Often an elastomeric material called underfill is applied between the die and the substrate to assist in relieving stresses that develop between the die and the substrate due to thermal mismatch.
Therefore, an important step in the interconnection of many electronic components is the formation of solder balls in the area array electronic package.
Heretofore, in this field, solder bumps or balls have been typically formed utilizing one of four methods: (1) printing of solder paste through a stencil or mask; (2) electroplating; (3) evaporation; or (4) mechanical transfer of preformed solder spheres. Electroplating, printing of solder paste through a stencil or mask, and evaporation techniques have been typically utilized for forming solder bumps on wafers and integrated circuits; flip-chip and chip scale packages have commonly utilized printing of solder paste and mechanical transfer of solder ball techniques.
Transfer of solder balls has been customarily achieved by means of vacuum chucks or machined templates. Another method for transferring preformed solder balls utilizes formation of a pattern of dots onto a photoimageable coating laminated to an organic film. Typically the organic film is composed of a material having a high melting temperature that is capable of being exposed to temperatures exceeding 200 C. with very little degradation, such as polyimide.
The pattern is formed by placing a photomask on the coating and then exposing the coating to a dose of ultraviolet radiation. For example, for an area array package design, the photomask will contain a mirror image of the contact pads design. The areas protected by the photomask design retain their adhesiveness while the unprotected areas exposed to the ultraviolet radiation lose their adhesiveness. The array of adhesive areas corresponds to the pattern of contact pads found on the substrate, wafer or die to receive the solder connections.
After the adhesive areas are formed, solder balls are loaded onto the surface of the film and attached to the adhesive areas. The excess solder balls that lie on non-adhesive areas are removed. The populated film is then aligned and brought into contact with contact pads, which may be fluxed. A solder reflow is performed to transfer the solder balls from the adhesive areas to the contact pads of the substrate. Following the reflow cycle, the film is removed from the solder balls.
The process of forming a pattern of adhesive areas can be used to transfer both sets of solder spheres. The smaller, flip chip spheres to the wafer or die (flip chip) and the larger, CSP spheres to the interposer or substrate. However, two distinct processes are required with each procedure involving a sphere population step, alignment of the populated adhesive areas to the wafer or interposer and a solder reflow step.